The present invention relates to the field of flexible wiring boards, particularly to the field of board pieces constituting flexible wiring boards and flexible wiring boards formed by assembling the board pieces.
Flexible wiring boards having a desired circuit pattern printed thereon have been widely used, and in recent years, there are demands for flexible wiring boards having various shapes suitable for the contours of the place where they are used.
FIG. 10(a) shows a layout for cutting T-shaped flexible wiring boards 252 from a rectangular pre-cutted substrate 250, in which six flexible wiring boards 252 are obtained.
However, a significant part of pre-cutted substrate 250 is uselessly discarded when such shaped flexible wiring boards 252 are cut.
Thus, a technique for preparing a flexible wiring board in a complex shape was proposed by dissolving the complex shape into simple shapes and assembling board pieces in the A simple shapes. Reference 255 in FIG. 10(c) represents a flexible wiring board having the same shape as that of flexible wiring board 252 described above and formed by assembling two rectangular board pieces 253, 254.
When rectangular board pieces 253, 254 are used in this manner, a close layout can be achieved as shown in FIG. 10(b) and therefore, precut substrate 250 can be effectively used by cutting board pieces 253, 254 in simple shapes. In FIG. 10(b), eight board pieces each 253, 254 are obtained and assembled into eight flexible wiring boards 255. As shown in this example, a greater number of flexible wiring boards 255 can be obtained by using board pieces in simple shapes than directly cutting T-shaped flexible wiring boards 252.
When a plurality of board pieces are assembled into a flexible wiring board as described above, board pieces 253, 254 must be mechanically and electrically assembled.
An example of the assembling technique is explained with reference to FIGS. 11(a), 11(b), in which references 220, 230 represent board pieces having metal wirings 222, 232 consisting of a patterned copper thin film formed on polyimide films 221, 231, respectively. In order to assemble these board pieces 220, 230, boards elements 220, 230 are first opposed to each other with metal wirings 222, 232 facing each other.
Metal wirings 222, 232 have solder coatings 223, 233 formed by plating on their surfaces, respectively, and solder coatings 223, 233 of the respective board pieces 220, 230 are brought into close contact with each other and heat and pressure are applied to melt solder coatings 223, 233, which are then cooled to form a solder layer 204. This solder layer 204 forms a metallic bond with metal wirings 222, 232, whereby metal wirings 222, 232 are firmly connected to each other via solder layer 204 to give a flexible wiring board 203 (FIG. 11(b)).
However, the recent need for forming a large number of metal wirings on a small-area flexible wiring board leads to an increasingly narrower pitch between metal wirings 222 (or metal wirings 232) on the same board piece 220, 230.
When solder coatings 223, 233 are heated under pressure as described above, molten solder scatters and remains here and there as scatter 215 in flexible wiring board 203, and in extreme cases, molten solder flows out to form a bridge 216 at connection 212 between metal wirings 222, 232, which causes a short circuit between metal wirings 222 (or metal wirings 232) to be insulated.
As the pitch between metal wirings 222 (or metal wirings 232) becomes narrower, bridge 216 becomes more likely to occur. Especially when a plurality of board pieces are to be assembled into a flexible wiring board, even one short circuit at the connection between board pieces means a faulty flexible wiring board as a whole, which extremely lowers the manufacturing yield of flexible wiring boards.
A board piece of the present invention comprises a non-thermoplastic resin film, a thermoplastic resin film formed on the non-thermoplastic resin film and a metal wiring formed on the surface of the thermoplastic resin film.
In this board piece, the metal wiring can be partially covered with a resin film.
The metal wiring can also be partially exposed.
A low-melting metal coating can be formed on at least a part of the exposed metal wiring.
A solder can be used as a material for the low-melting metal coating.
A gold coating can also be formed on at least a part of the exposed metal wiring.
The non-thermoplastic resin film can consist of a polyimide film.
The thermoplastic resin film can consist of a thermoplastic polyimide film.
A flexible wiring board of the present invention comprises at least two board pieces each having a non-thermoplastic resin film, a thermoplastic resin film formed on the non-thermoplastic film and a metal wiring formed on the surface of the thermoplastic resin film, wherein the board pieces are adhered to each other via the thermoplastic resin films by heating the metal wirings of the board pieces in contact with each other to soften the thermoplastic resin films and insert them between connections of the metal wirings in contact with each other.
In the flexible wiring board, a low-melting metal coating can be formed on the surface of at least one of the metal wirings in contact with each other so that the board pieces are heated to melt the low-melting metal coating, which then solidifies to connect the metal wirings to each other.
The metal wirings in contact with each other can also be connected by ultrasonic wave vibration.
In this case, a gold coating is preferably formed on at least one of the metal wirings to be connected by ultrasonic wave vibration.
Another flexible wiring board of the present invention is formed by assembling at least three board pieces each having a resin film and a metal wiring, wherein the metal wirings of a first and second board pieces among the board pieces are partially connected and fixed to the metal wiring of a third board piece and a thermoplastic resin is filled between the connecting and fixing regions of the metal wirings while the other regions of the metal wirings are partially covered with a protective film on their surfaces and partially exposed.
In the flexible wiring board, the resin film of at least one of the board pieces to be assembled has a multilayer structure consisting of a non-thermoplastic resin film and a thermoplastic resin film and the metal wiring can be formed on the thermoplastic resin film.
The board pieces can be connected to each other by applying a thermoplastic resin film on each of the metal wirings and then assembling the board pieces with a thermoplastic resin constituting the thermoplastic resin film being filled between the connecting regions of the metal wirings.
The metal wirings can be fixed to each other with a solder.
An opening can be formed in the protective film provided on at least one of the first to third board pieces.
In a process for manufacturing a flexible wiring board of the present invention by assembling a plurality of board pieces having a resin film and a metal wiring provided on the resin film, the resin film of at least one of the board pieces to be assembled has a multilayer structure consisting of a non-thermoplastic resin film and a thermoplastic resin film and the metal wiring is provided on the thermoplastic resin film and the board pieces to be assembled are pressed against each other under heating.
In this case, the metal wirings of the board pieces can be connected to each other by forming a low-melting metal coating on the surface of the metal wiring of at least one of the board pieces to be assembled and pressing the board pieces against each other under heating to melt the low-melting metal coating.
The board pieces can be assembled by pressing the board pieces against each other under heating after applying ultrasonic wave to the board pieces in a superposed state to connect the metal wirings of the board pieces to each other by vibration energy of the ultrasonic wave.
The board pieces can also be assembled by pressing the board pieces in a superposed state against each other under heating while applying ultrasonic wave to connect the metal wirings to each other by vibration energy of the ultrasonic wave.
In this case, a gold coating is preferably formed in advance on the surface of at least one of the metal wirings to be connected.
In another process for manufacturing a flexible wiring board of the present invention by assembling a plurality of board pieces having a resin film and a metal wiring provided on the resin film, the metal wirings of the board pieces are connected to each other by providing a thermoplastic resin film between the metal wirings of the board pieces to be assembled and pressing the board pieces against each other under heating to force the metal wirings into the thermoplastic resin film.
In this case, the metal wirings of the board pieces can be connected to each other by forming a low-melting metal coating on the surface of the metal wiring of at least one of the board pieces to be assembled and pressing the board pieces against each other under heating to melt the low-melting metal coating.